Aero Precision M4E1 Stripped Lower Receiver

$155

Standard AR-15 lower for all five calibers. 7.62x39 steel mags can bind or fail to lock back in some magwells; an enhanced bolt catch or specific mag brand may be needed. .450 BM uses dedicated straight-wall mags that fit a standard magwell.

Build with this part

Opens the builder with this part fitted. Other parts change only if they have to.

Pros and cons

Pros

Threaded bolt catch pin, no hammering needed
Upper tension screw reduces wobble
Integrated trigger guard and flared magwell

Cons

Bolt catch pin uses proprietary threads
Heavier than some forged lowers (8.4 oz)

Builds using this part

.300 Blackout suppressed build $2,302
16" general-purpose rifle with LPVO $2,097
Budget home-defense carbine $1,404
MK18-style duty carbine $2,768

Other lower receiver options

Geissele Automatics Super Duty Stripped Lower Receiver $175
Spike's Tactical Spider Stripped Lower Receiver $107
Centurion Arms CM4 Forged Stripped Lower Receiver $135
Palmetto State Armory Sabre-15 Ambi Lower Receiver $100
Griffin Armament Mk2 Ambidextrous Stripped Lower Receiver $220
American Defense Manufacturing UIC Ambidextrous Stripped Lower Receiver $370
Radian Weapons AX556 Ambidextrous Lower Receiver $450
Aero Precision AR15 Gen 2 Stripped Lower Receiver $120